Create hollow intersecting tubes
In User Preferences search for the "Pipe Joints" addon and enable it.



Press ShiftA and go to Mesh > Pipe Joints > Pipe Y-Joint.



UPDATE:

Pipe Joints is now part of the Extra Objects Addon in the Add Mesh category


And can be accessed through Add>Mesh>Mechanical>Pipe Joints



You can control the shape and angle of the joint by using the tab on the left side of the 3D view.



To add some thickness use a solidify modifier.